The Biochemistry of Alactic + Aerobic Training - Muscle Fibers and How They Use Energy


[This is part 6 of many about the details of Alactic + Aerobic training and why you should be doing it.]

I'm inserting an extra segment in this series. I was going to tie everything together after the discussion of the 3 energy systems, but I just found Al Ciampa's blog (here) and he has a whole section on muscle fibers and how they interact with the energy systems. I found it fascinating and it brings even more clarity to the A+A discussion. So with apologies to Al, I'm going to steal a lot in this section from him - he's the A+A guru anyway.

I think it's safe to conclude that there are three discrete energy systems in the body. Each one has a characteristic lifetime and a characteristic speed.

  • Alactic, high power, short duration

  • Glycolytic, medium power, medium duration

  • Aerobic, low power, long duration

The power output is basically determined by chemical kinetics (untrainable, fundamental nature of things), and the duration is likely determined by what chemicals available for the reaction (e.g, when your run out of Creatine Phosphate, you no longer have Alactic power available).

So how do we "train energy systems" if it's all determined by fundamental chemical properties?

That's where the discussion of muscle fibers starts to matter. It's always bothered me that we categorized fibers into "slow twitch" and "fast twitch" also and then got all fancy and called them Type I and Type II and then started to subdivide into Type IIA and Type IIB. I just never paid that much attention to it because it all felt like we were just naming things without really understanding them. Al's take on this is that it's much more likely that there are a continuum of fiber "types" that are on a spectrum from slow to fast:

slowest -> slower -> slow -> fast -> faster -> fastest

What distinguishes muscle "type" is the ratio of the content of each energy system inside each fiber. Slower fibers are more aerobic with all that that entails: low power, long duration. Faster fibers are more anaerobic: high power, short duration.

The only way you can control which fiber that's used in an activity is the intensity of the activity. Fibers are recruited in order from slowest to fastest. So if you do a low-power activity, only the slow fibers are used. If you do a high-power activity, slow plus enough faster fibers are used until they generate enough force to do what you need. That is, you can't selectively use the fast ones over the slow ones, you use the slow ones plus enough of the fast ones until you get the force you need.

So then, if you call an activity "aerobic" or "anaerobic" is just a continuum based on which fibers are used. This gives the result we're used to, long-slow-distance is aerobic (mostly), and high speed or heavy lifting (or both if you do CrossFit) will have a larger anaerobic contribution.

I think this explains much of what we intuitively know, but have a hard time really believing. Why does sprinting help your endurance, but not the other way around? Why can't you get strong with high reps of light weight? Why does squatting a ton help your cycling, but not the other way around?

When you do the low-power, low-intensity work you're never at a threshold to recruit the "faster" fibers. But when you do the high-power, high-intensity work, you bring in the slow and the fast. The slow are always involved!

Here's how Al described it:

If you begin mall-walking, the slowest fibers are recruited and the activity is mostly an aerobic one; however, to actually get your started, there is an anaerobic “boost”

If you pick up the pace to a semi-brisk walk, there may or may not be a boost of anaerobic metabolism based on the strength and endurance of your slower muscle fibers (in the extremely detrained or untrained, this already can be a mostly anaerobic activity)

If you push the accelerator to the floor of your walking gait, chances are your slower fibers and so the aerobic pathway can cover this action

If you break into a slow jog, the sharp increase in total force production required will no doubt enlist faster fibers, increasing anaerobic pathway involvement

Depending upon your training state and the duration of movement at this point, your slower fibers might be able to cover down on your jog

If you continued to jog at this slow speed, when your slower fibers powering this activity fatigue, faster fibers will cover down, causing the activity to be more and more anaerobic as you move forward (this can be seen on a heart rate monitor, as it begins to grow higher and higher)

If, however, your slower fibers can endure, you will remain mostly aerobic

If you hit hills or turn into the wind, and want to maintain the same speed, faster fibers will be recruited, temporarily increasing anaerobic metabolism

If you speed up, faster fibers again must be recruited to cover the additional force production cost

At any point that reduce the force production required, i.e., you slow down; and your slower fibers are not fatigued, you will be more and more aerobic

And this will go on and on…

And in the context of a higher power activity - the 5-minute snatch test:

The faster fibers (but probably not the fastest) will be recruited, making this a wholly anaerobic event

As these faster fibers fatigue, the fastest fibers will begin to be recruited

As these fibers fatigue, the only fibers left are slower; making the event more and more aerobic as time goes on

If these slower fibers are not very strong, you may have to stop the event all together; if they are, however, your pace and intensity will simply have to decrease

If you pace this properly, the fastest (or fast”er”) fibers might be able to recover enough during the activity, which allows for a sprint to the finish!

In all cases, the "speed" of the workout will determine the "aerobicness" of the workout. If you slow down, you will be more aerobic. That's good - it's the clean burning pathway. But, if you only do aerobic work you'll never use the other fibers, and what you don't use won't be maintained by your body. So you need to hit those high-power fibers to tell your body to keep them around. That means everyone needs to lift heavy and move fast!

You adjust the dose not by changing the intensity - you need to move fast and heavy - but by changing the duration. You only move fast and heavy long enough to get the stimulus, not long enough to get the acid load from the sugar burning.

Michael Deskevich